mtd: nand_bbt: scan for next free bbt block if writing bbt fails

If erasing or writing the BBT fails, we should mark the current BBT
block as bad and use the BBT descriptor to scan for the next available
unused block in the BBT. We should only return a failure if there isn't
any space left.

Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Suggested-by: Jeff Westfahl <jeff.westfahl@ni.com>
Tested-by: Kyle Roeschley <kyle.roeschley@ni.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>